ISO Tracker

The ISO-tracker™ app allows the user to seamlessly connect to the Bluetooth device (iso-tracker.com) to see in real-time:
Their current force output.
The max force output reached.
The average force exerted during the session. (You can choose from two different modes with a tap of the Average button. The default setting tracks the average force over the course of the hold time. Swap to Average+ to have the app count reps, instead.)
Also, auditory feedback lets you know when you've reached your set target force, as well as when you've reached the target hold time/reps.
The app will read in Lbs, as well as Kgs.